1. Field of the Invention
The present invention relates to an illumination device capable of efficiently illuminating an illumination area on an object to be illuminated and an inspection system having such an illumination device.
2. Description of the Related Art
Conventionally, photographing apparatuses such as video cameras and digital cameras are equipped with an illumination device that illuminates a subject to be photographed with light emitted from a light source when the brightness of the subject is insufficient. Such an illumination device has a light source and an optical element such as a reflecting mirror or a Fresnel lens that efficiently guides light beam emitted from the light source frontward (i.e. toward the subject to be photographed).
Various types of such illumination devices that are adapted to efficiently guide light beams emitted from the light source in variety of directions toward a subject to be photographed have been developed.
For example, Japanese Patent Application Laid-Open No. H04-138438 discloses an illumination device that illuminates an object to be illuminated with light beams emitted in the forward direction from the light source through a small prism having high condensing efficiency.
As another example, Japanese Patent Application Laid-Open No. H08-234277 (corresponding to U.S. Pat. No. 6,058,271) discloses an illumination device in which an optical member in the form of a light guide is provided in the vicinity of a light source to illuminate a surface to be illuminated with light beams emitted from the light source while condensing the light beams and making the distribution of light uniform without a significant light loss.
As another example, Japanese Patent Application Laid-Open No. H08-262537 (corresponding to U.S. Pat. No. 5,813,743) discloses an illumination device in which a prism is disposed in front of the light source in an inclined manner asymmetrically with respect to the optical axis of illumination in order to reduce the entire size of the apparatus.
On the other hand, Japanese Patent Application Laid-Open No. 2005-086391 and Japanese Patent Application Laid-Open No. 2004-177523 disclose illumination devices using a corner cube to effectively make use of light emitted from a light source that could otherwise go outside the illumination optical system.
In conventional illumination devices, a reflecting mirror is provided in rear of the light source (i.e. at the opposite side of the light source with respect to the object to be illuminated) to enhance the illumination efficiency.
In such illumination devices, if the positions of the light source and the reflecting mirror are displaced from the designed positions, the optical path of the light reflected by the reflecting mirror changes, which makes impossible to efficiently illuminate the object to be illuminated.
Japanese Patent Application Laid-Open No. 2004-177523 teaches to enhance the utilization efficiency of illumination light directed to an object to be illuminated by using a corner cube disposed in front of the light source (i.e. at the object-to-be-illuminated side of the light source).
However, in the arrangement disclosed in Japanese Patent Application Laid-Open No. 2004-177523, since the rotary parabolic mirror is provided in rear of the light source, highly accurate alignment of the light source and the rotary parabolic mirror is required, and misalignment therebetween will make it difficult to enhance the illumination efficiency.